Final answer:
Human placental lactogen (hPL) mainly inhibits insulin action during pregnancy to increase glucose availability to the fetus, and it also assists in preparing the mammary glands for future lactation.
Step-by-step explanation:
The function of human placental lactogen (hPL), also known as human chorionic somatomammotropin (hCS), is complex and involves several physiological effects during pregnancy. However, considering the available options, the most accurate answer to the given function is that hPL inhibits insulin action. This hormone, produced by the placenta, is structurally similar to human growth hormone and plays a role in modulating the metabolic state of the mother during pregnancy to facilitate the energy supply of the fetus. hPL acts to reduce maternal insulin sensitivity, which helps to increase glucose availability for the fetus. It is important to note that hPL also has other functions, including promoting mammary gland growth in preparation for lactation, but it does not directly stimulate milk production; that role is primarily played by prolactin.